This invention relates in general to apparatus for coupling data between stationary and rotary systems. More particularly this invention relates to fresnel lens apparatus for optically coupling a plurality of data channels between the rotating and stationary structure of the rotary head scanner of a magnetic tape recorder.
Advanced magnetic tape recording and reproducing systems require ever higher data transfer rates and increased bandwidths. In helical scan magnetic tape recorders using rotary head scanners, there must be some means for transmitting signals between the rapidly rotating head wheels upon which the recording and reproducing heads are mounted and stationary signal processing circuitry. Typically, rotary transformers are used to transmit both the record and reproduce signals from and to the magnetic heads carried by the rotating head wheel. Rotary transformers, however, have a predicted upper frequency limit of approximately 150 megahertz. Moreover, at such high signal frequencies, dimensional tolerances between the stationary and rotary coils of the rotary transformer are severe and cross talk between adjacent rotary transformers is difficult to eliminate.
In order to mitigate the disadvantages of rotary transformers, several proposals have been made to optically transmit signals between the stationary and rotating structure of a rotary head scanner. Optical transmission has an upper frequency limit of six gigahertz at the present time, with an unknown limit in the future. Moreover, crosstalk between record and reproduce signals and crosstalk between adjacent channels is virtually eliminated and dimensional tolerances are less severe. Where only one or two signals (for example, record and reproduce signals) are to be optically transmitted between the rotating and stationary structure of a rotary head scanner, the simplest technique is to have the optical signal transmission axis coincide with the axis of rotation of the head wheel or to have, at least either the optical signal transmitter or receiver coincident with the axis of rotation of the rotating head wheel. Such a technique is disclosed, for example, in Japanese Kokai Patent No. 53-21912, published Feb. 28, 1978, Inventors, Koshimoto et al.; in U.S. Pat. No. 4,401,360, issued Aug. 30, 1983, Inventors Streckmann et al.; and in U.S. Pat. No. 4,511,934, issued Apr. 16, 1985, Inventors Ohira et al. The arrangements disclosed in these patents are generally not easily adaptable to the transmission of multiple optical signals over separate optical paths. In order to minimize interference between the signals, either a half mirror or filter is used to separate two optical signals transmitted over the same optical axis which coincides with the axis of rotation of the head wheel.
Although multiplexing techniques (for example, time division multiplexing, frequency division multiplexing or wavelength division multiplexing) may be used to optically transmit multiple signals along the same optical axis, the circuitry required to multiplex and demultiplex such signals is complex and costly and susceptible to crosstalk and signal degradation. Thus, it is desirable to optically transmit each signal along a separate optical path in order to minimize such difficulties. Several techniques have been proposed for providing optical transmission between a first array of optical elements mounted on a rotating body and a second complementary array of optical elements mounted on a stationary or rotating body. Thus, in U.S. Pat. No. 4,447,114, issued May 8, 1984, Inventor Koene, there is disclosed an optical coupling body which is disposed between first and second arrays of optical conductors mounted for rotation at equal but opposite rates relative to the optical coupling body. Either light reflective or light transmissive means is arranged in the body to effect constant coupling of individual conductors in the first array with respective conductors in the second array. A similar technique is disclosed in U.S. Pat. No. 4,109,998, issued Aug. 29, 1978, Inventor Iverson and U.S. Pat. No. 4,258,976 issued Mar. 31, 1981, Inventors Scott et al. As disclosed, a derotation assembly is located between a stationary body and a rotating body having respective complementary arrays of optical transmitting and optical receiving elements. The derotation plate is rotated at half the speed of the rotating body. The optical transmission arrangements of the latter three patents are disadvantageous because of mechanical and electrical complexity, cost, and unreliability.
U.S. Pat. No. 4,519,670, issued May 28, 1985, Inventors Spinner et al., discloses a light rotation coupling for the transmission of a plurality of light channels between two parts which rotate relative to each other. A plurality of radially arrayed light transmitters rotate about an axis which coincides with the optical axis of a multiple refractive or reflective light transmitting optical assembly. A plurality of light receivers are axially arrayed along the optical/rotation axis. This technique is disadvantageous because of the use of complex, expensive and heavy optical transmission assemblies. Moreover, locating the light receivers on the axis of rotation is disadvantageous for several reasons. First, the coupling system is bulky and not suitable for applications where space is at a premium. Moreover, bidirectional transmission of data is difficult because the optical data channels are interfered with by the electrical conductors and hardware associated with the other optical data receivers. Thus, data cannot be transmitted at all times.
In another proposed optical signal transmission technique, individual optical slip rings are stacked along the axis of rotation of a moving body. Thus, U.S. Pat. No. 4,278,323, issued July 14, 1981, Inventor Waldman, discloses an optical signal transmission system which includes a plurality of axially spaced optical fiber rings rotatably mounted on a spindle. The rings are mounted in a fixed casing having a number of separate chambers corresponding to the number of optical fiber rings. Fixed fiber optic blocks surround each fiber optic ring. Complementary LEDs and photodiodes are respectively embedded in each optic fiber ring and block to effect optical signal transmission between the stationary optic fiber block and the rotating optical fiber ring. In U.S. Pat. No. 4,444,459, issued Apr. 24, 1984, Inventor Woodwell, a fiber optic slip ring comprises a toroidal optic wave guide which is uncoated along a circumferential window extending along the inner or outer circumference of the wave guide. An electronic device for transmitting an optical signal is connected to one end of the wave guide. A ring of radiation is produced so that an optical signal receiver may be positioned at any location around the circumference of the wave guide to receive the transmitted optical signal. As disclosed in the latter patent, a plurality of slip rings may be arrayed along the rotational axis of the receiver in order to effect transmission of a plurality of separate optical signals. The optical slip ring systems of the latter two patents are disadvantageous, among other reasons, because of the inefficiency in producing a 360.degree. ring of radiation.